Nowadays, with the rapid development of construction industry and the continuous improvement of material design, a higher requirement to admixture is put forward. In recent decades, some new technologies, including large template, sliding mould, pumping concrete, vacuum dehydration concrete, shotcrete and so on, increasingly appear in the construction field. Furthermore, the commercial concrete and batch mixing method appear in the supply field of concrete, as well as in the field of structural engineering, there are high-rise, super high-rise, large span, folded plate, thin shell, framed light-weight panel system, shear wall system, assembly structure, box structure, unbounded prestressed concrete system, and frame-tube system occurred. With the rising of these new technologies, new methods, and new structures, the higher and newer requirements for concrete's construction performance and economic indicators are put forward. For instance, the modern concrete requires greater performance involving liquidity, plasticity, frost resistance, impermeability, compaction density, retarding, rapid hardening, high strength, and so on. Therefore, the concrete admixture used in the past cannot meet the requirement of modern construction. Moreover, a higher requirement is put forward for each component of the green and high-performance concrete, especially for concrete admixture.
The continuous development of design and synthesis for molecular structure in polymer subject provides the theoretical basis for us to “repair” an ideal molecular structure. In recent years, star-shaped polymer is paid more and more attention by researchers because of its good solubility, low viscosity, multi-function, high reactivity and a larger volume space. Consequently, a star-shaped polycarboxylate superplasticizer was designed and synthesized by copolymerizing the conventional comb-shaped polymers with the active “cores” which contained a large number of polymerizable ends. The characteristic of multi-arm for this star-shaped copolymer can ensure more effective stereo-hindrance effect. The great application potential can be shown for it exhibits not only the performance of conventional comb-shaped polymer but also the special nature of star-shaped polymer.
The patent CN102199298A (publication date: Sep. 28, 2011) reported a preparation method of comb-shaped polycarboxylate surfactant. This invention stated a comb-shaped surfactant synthesized through three steps: the first intermediate polymer copolymerized between acrylic acid and its ester monomer in the presence of initiator and flowing nitrogen at the set heating temperature; the second grafting reaction by adding the grafting compound, catalyst and solvent; and the last product precipitation by using precipitating agent after removing the solvent by reducing pressure. This invention has the advantages of reasonable technology, simple operation, low preparation cost, mild reaction conditions and good product quality, but still belongs to a traditional comb-shaped polycarboxylate product without star-shaped polymer's some unique advantages. Therefore, the qualitative leap and breakthrough cannot be achieved essentially for the structure of polycarboxylate surfactant.
The patent CN1957006A (publication date: May 2, 2007) reported a preparation method of star-shaped acrylic polymer. The objective of this invention was to provide a structure-controllable star-shaped acrylic polymer with a narrow molecular-weight distribution and acid decomposability, which had the polyacrylate-derived polymer chain as “core” part, as well as the (meth)acrylate-derived polymer chain as “arm” part. This patent disclosed a novel star-shaped polyacrylic derivative without describing application fields. It is hard to be applied in the field of concrete admixture because the superplasticizer's side-chain-like stereo-hindrance effect is difficult to be achieved without using polyethylene glycol compounds as reactants. Besides, the complicated synthesis process, strict reaction conditions, high polymerization cost and insufficient popularizing prospect, together with the obtained star-shaped polycarboxylate superplasticizer without an ideal structure, imply an unknown application performance.
The patent CN101585900A (publication date: Nov. 25, 2009) reported a preparation method of concrete polycarboxylate superplasticizer. This method had two steps including an esterification and a polymerization. Typically, the esterification was carried out by using the methoxy polyethylene glycol (MPEG) and meth acrylic acid as main reactants and adding polymerization inhibitor and catalyst. When the esterification finished, the esterification product was diluted to a concentration of 73% and adjusted to pH value of 3.5-4.0, followed by adding mercaptopropionic acid and then cooled to 43° C. The resultant and demineralized water as the ratio of 20:8 were pumped into two elevated tanks respectively and then the aqueous solution of initiator was uniformly dropped into reactor followed by polymerization for 4 hours at constant temperature. After cooling, the pH value was adjusted by adding alkali liquor, and finally the concrete polycarboxylate superplasticizer was obtained. This invention solved the sensitivity problem of superplasticizer's dosage, and the produced polycarboxylate superplasticizer exhibited the low cost, high water-reducing ratio, good adaptability to cement and easy industrialized production. Although the polycarboxylate superplasticizer with comb-shaped structure can be achieved by both esterification and polymerization, it still belongs to the traditional comb-shaped structure's category, and didn't achieve the improvement of application performance only simply by changing synthetic process but not by designing new-type molecular structure.
The concrete superplasticizers or surfactants described in the most patents have already good workability such as fluidity and adaptability to cement. However, the preparation methods described above all have some disadvantages, involving that the synthesized polycarboxylate superplasticizers still have the traditional comb-shaped structures, and the excellent application performances in cement or concrete are achieved mostly through researchers' changing reaction materials or process conditions but seldom through changing polymer structure from the design of molecular structure. Some star-shaped acrylic polymer are not referenced or applied in the field of concrete admixture, but it is a fact that increasing the polymer density is beneficial to distinct improvement in polymer's action efficiency and application performance. Hence, it is required that the synthetic polycarboxylate superplasticizers do not only change the traditional comb-shaped structure into the more efficient star-shaped structure to further achieve the excellent workability, but also ensure a simple process operation and low preparation cost to easy to industrialize. The related research has not been reported in domestic and abroad.